The present invention relates to methods of manufacturing a group III-V compound semiconductor of AlInGaN for use in a light emitting device having a wavelength region from UV to orange-colored light, a semiconductor device using the group III-V compound semiconductor and a GaN semiconductor substrate.
Recently, there have been increasing demands for a light emitting diode for emitting visible light in a range between blue and red to be used as a light source for a multicolored display. In particular, various studies have been made on a group III-V compound semiconductor light emitting diode of gallium nitride (GaN) operated in a wavelength region from UV to orange-colored light. In this light emitting diode, an active layer is generally formed out of indium gallium nitride (InGaN), and the most significant point in realizing this light emitting diode is attaining improvement of the quality of the crystal by decreasing defects in the InGaN layer.
Now, a conventional method of growing a semiconductor of InGaN will be described.
The conventional method of growing a semiconductor of InGaN is described in, for example, Japanese Laid-Open Patent Publication No. 6-209122. This publication describes that it is significant to control both a growth temperature and a growth rate in order to obtain InGaN having high quality and good crystallinity at high reproducibility, and that nitrogen is preferably used as a carrier gas for a material gas.
Also, as first paper relating to composition control of In in InGaN crystal, "Applied Physics Letters, Vol. 68 (1996) pp. 3147-3149" will be given. According to this paper, it is effective to decrease the growth temperature and increase the growth rate in order to increase the composition ratio of In in InGaN.
Furthermore, as second paper examining comparison of plural GaN light emitting diodes provided with InGaN active layers respectively including different composition ratios of In, "Japanese Journal of Applied Physics, Vol. 34 (1995) pp. L797-L799" will be given. According to this paper, when the composition ratio x of In in the active layer of In.sub.x Ga.sub.1-x N (wherein 0&lt;x&lt;1) is gradually increased, for example, so that a blue light emitting diode in which the composition ratio x of In is 0.2 and a yellow light emitting diode in which the composition ratio x of In is 0.7 can be compared with each other, the yellow light emitting diode is inferior to the blue light emitting diode in the luminance efficiency, the color purity and the luminance.
This is for the following reason: As the composition ratio of In in the active layer is increased, a difference in the lattice constant and the thermal expansion coefficient between the active layer and a barrier layer of, for example, AlGaN disposed in the vicinity of the active layer is increased, resulting in causing excessive strain in the active layer. As a result, when this strain is increased to have a value exceeding a critical value, a defect such as misfit dislocation is introduced into the active layer so as to relax the strain, and hence, the luminance of the light emitting diode is degraded.
In this manner, although the above-described publication and the first paper describe the method of controlling the composition ratio of In in a semiconductor of InGaN, they do not mention the quality improvement of InGaN crystal when the composition ratio of In is large as in a yellow or orange-colored light emitting diode. Therefore, there arises a first problem that InGaN crystal having high quality and including less defects cannot be obtained when the composition ratio of In is large. According to an opinion described in the second paper, it is difficult to obtain InGaN crystal including a large composition ratio of In and having good crystallinity merely by controlling the growth temperature and the growth rate.
Also, in the case where a GaN semiconductor is doped to achieve a p-conductivity, and for example, a resistance is required to be decreased as in a p-type contact layer, when magnesium (Mg) used as a p-type dopant is excessively supplied in the growth of the semiconductor, there arises a second problem that the resistance of the p-type contact layer is increased in contrast or the p-type contact layer is changed to have an n-type conductivity.
Furthermore, since a good quality GaN substrate is not available for use in a GaN semiconductor, sapphire (Al.sub.2 O.sub.3) is occasionally used as a substrate. Sapphire has an insulating property. In the case where a conductive substrate is used, for example, in manufacturing a light emitting diode, it is easy to form one electrode on a device forming area and the other electrode on the other surface of the device forming area. In the case where sapphire is used, however, it is necessary to form one electrode on a device forming area and to remove a part of the device forming area to form the other electrode. In this manner, there arises a third problem that the manufacturing process is complicated.